In recent years, electric and hybrid vehicles driven by an alternate current (AC) motor are the focus of attention in terms of their advantageous features that match a social demand for low fuel consumption and low exhaust gas emission.
For instance, in some electric vehicles, the AC motor and a direct current power source of a secondary battery are connected by way of a motor controller, which includes an inverter. The AC motor is driven by converting a direct current voltage of the direct current power source into an alternate current voltage with the inverter. In some hybrid vehicles, two AC motors and the direct current power source of the secondary battery are connected by way of the motor controller, which includes the inverter, to drive the AC motors by converting a direct current voltage of the direct current power source into an alternate current voltage with the inverter.
The control systems of such AC motors in the electric and hybrid vehicles detect two or more phase electric currents in the three-phase AC motor, by two or more electric current sensors. Based on the electric current detected from the sensors, the AC motor is controlled. However, by having many electric current sensors for one AC motor, a reduction of the size, volume, and cost of the three phase output terminals as well as the motor control system as a whole is hindered.
An effort to reduce the cost of the AC motor control system is proposed in, for example, Japanese Patent Laid-Open No. 2001-145398 (patent document 1), which is U.S. Pat. No. 6,229,719. Patent document 1 discloses reducing the number of electric current sensors by detecting the electric current of one of three phases in the AC motor (i.e., U phase) via one current sensor. In particular, based on the electric current detected in one phase (i.e., U) by the current sensor and the electric current estimate values of the other two phases (i.e., V, W phases) in a previous cycle, a d-axis electric current estimate value (i.e., an excitation-origin electric current estimate value) and a q-axis electric current estimate value (i.e., a torque-origin electric current estimate value) are calculated. Based on the smoothed values of the d-axis electric current estimate value and the q-axis electric current estimate value, which may be averaged (i.e., smoothed) by a first-order delay filter, the electric current estimate values in other two phases are also calculated, for controlling the AC motor by using the q-axis electric current estimate value (or, using both of the d-axis electric current estimate value and the q-axis electric current estimate value).
By having one electric current sensor for one AC motor (i.e., a single-phase sensing of electric current), the technique of patent document 1 is able to reduce the cost of the inverter as well as the volume or size. However, in the event of a motor torque change, which that causes, for example, a steep change of the actual electric current, the responsiveness of the electric current estimation may not be sufficient, thereby causing a delay of a catch-up of the electric current estimate value. The delay of the electric current estimate value may further lead to a torque variation or fluctuation due to the estimation error and to a lack of torque responsiveness of the AC motor, which is a required and important feature of the AC motor.